Test mode: can't test motors if part of RobotDrive

[fovea1959]

Put together a minimal command based robot with RobotBuilder, just three motor controllers, two subsystems, no commands.

Two of the MotorControllers are part of a RobotDrive in the DriveSubsystem, the other MotorController is in the other subsystem.

When we go into test mode, we can work the single drive motor just fine with the slider in the SmartDashboard; but the motors that are part of RobotDrive mostly stay off, with erratic and brief attempts to turn on. It's almost as if there are multiple threads or commands working the motor controllers there.

Did I uncover a bug (or rediscover a known one that I can't find a reference to)? I put this project together just as a test case for this when we discovered the erratic behaviour in a larger project.